Multi-radiator antennas are frequently used in for example cellular networks. Such multi-radiator antennas comprise a number of radiating antenna elements for example in the form of dipoles for sending or receiving signals, an antenna feeding network and an electrically conductive reflector. The antenna feeding network distributes the signal from a common coaxial connector to the radiators when the antenna is transmitting and combines the signals from the radiators and feeds them to the coaxial connector when receiving. A possible implementation of such a feeding network is shown in FIG. 1.
In such a network, if the splitters/combiners consist of just one junction between 3 different 50 ohm lines, impedance match would not be maintained, and the impedance seen from each port would be 25 ohm instead of 50 ohm. Therefore the splitter/combiner usually also includes an impedance transformation circuit which maintains 50 ohm impedance at all ports.
A person skilled in the art would recognize that the feeding is fully reciprocal in the sense that transmission and reception can be treated in the same way, and to simply the description of this invention only the transmission case is described below.
The antenna feeding network may comprise a plurality of parallel substantially air filled coaxial lines, each coaxial line comprising a central inner conductor at least partly surrounded by an outer conductor with insulating air in between. The coaxial lines and the reflector may be formed integrally with each other. The splitting may be done via crossover connections between inner conductors of adjacent coaxial lines. In order to preserve the characteristic impedance, the lines connecting to the crossover element include impedance matching structures. The substantially air filled coaxial lines may be provided with a dielectric element to provide a phase shifting arrangement. The phase shift is achieved by moving the dielectric element that is located between the inner conductor and the outer conductor of a coaxial line. If the dielectric element is moved in such a way that the outer conductor will be more filled with dielectric material, the phase shift will increase. WO2009/041896 discloses an antenna arrangement provided with an adjustable differential phase shifter using such a movable dielectric element.
The radiating element is typically a dipole. A dipole usually may consist of two radiating parts having an electrical length of approximately one quarter of a wavelength at the operating frequency and extending essentially in plane parallel with the antenna reflector, and positioned approximately at a distance equivalent to one quarter of a wavelength at the operating frequency. The radiating parts are fed in counter-phase. Such a feeding is achieved by using a balanced-unbalanced transformer, also called a balun. In a dipole, it is often convenient to also use the balun as a mechanical support of the two radiating parts. The balun is often also used as an impedance matching element.
The balun consists of a body part and a coupling element which can also be seen as a conductor positioned in the centre of a cylindrical hole in the body part. The balun coupling element is electrically connected at one end to one of the radiating elements, and at the other end to a feeding line inner conductor.
The body part is usually connected to feeding line outer conductor and to the antenna reflector.
The connection between the radiating element and one of the inner conductors may be achieved using for example a screw joint. Thus, direct contact between the electrically conductive coupling element of the radiating element and an electrically conductive portion of the inner conductor is established. Such an arrangement has the disadvantage that it may be difficult and time consuming to assemble or manufacture since a screwed connection may be difficult to achieve in the very limited space available inside the outer conductor. Also, the screw and the coupling element are often inserted from opposite sides of the antenna which makes assembly difficult. Another disadvantage with the screw joint is that it may introduce passive intermodulation (PIM). Due to the small dimensions of the coupling element of the radiating element, the screw joint also needs to be of small dimensions, which makes it particularly difficult to achieve a connection which is sufficiently firm to avoid PIM.